1. Field of the Invention
The present invention relates generally to a mobile projector mounted on a mobile communication terminal, and more particularly to a method of controlling the power consumption of a light source such as Light Emitting Diodes (LEDs) used in the mobile projector.
2. Description of the Related Art
Recently, in order to overcome limited display size, a mobile communication terminal has been developed to have a TV-OUT function and a function to display information of the mobile communication terminal on an external large-scale display device by connection to an external projector. Also, a mobile projector technique that mounts a subminiature projector module on the mobile communication terminal instead of the external projector has been developed.
In general, a mobile projector uses LEDs as a light source. Also, in order to control the power of the LEDs, a circuit as illustrated in FIG. 1 is used in conventional systems.
Referring to FIG. 1, if light is emitted from the LED as current flows to the LED (not shown), an mPD (monitor Photo Diode) senses the output light and generates a corresponding photocurrent. As the photocurrent flows to a feedback resistor R, a voltage is generated across the feedback resistor R. This voltage is input to a feedback (FB) terminal of an Automatic Power Control (APC) LED driver 110, and is compared with a reference voltage, so that an LED driving current is increased or decreased to uniformly control the intensity of power output from the LED. This is generally called an APC drive.
However, when using the above-described conventional LED power control system for a mobile projector, several problems may occur, as described with reference to FIGS. 2A and 2B.
Referring to FIGS. 2A and 2B, the LED has characteristics that, if an operation environment temperature increases, the LED light power is reduced. Particularly, for a red LED among Red, Green, and Blue (RGB) light sources used for the mobile projector, the light power is abruptly decreased as operation temperature increases, resulting in a reduction of the light power of about −8% per 10° C. Accordingly, if the red LED is driven by the APC, it is necessary to increase the driving current to compensate for the light power that is reduced due to temperature increase. FIG. 2A shows the LED power consumption according to the operation environment temperature of the LED. Increased driving current for the above-described reasons causes additional heat generation of the LED, and when a conventional power control circuit is applied to the mobile projector, which rarely has a sufficient heat sink to dissipate the additional heat, the increase of the driving current brings an unacceptable increase in heat generation, with an associated temperature increase. As a result, a vicious cycle exists of increasing the driving current to compensate for reduced light power caused by temperature increase (i.e. increased driving current→increased heat generation→temperature increase→increased driving current), and thus the system may fail due to thermal runaway.
Also, if operation time is lengthened, the light power of the LED is reduced due to a gradual degradation, causing increased driving current by the APC operation. FIG. 2B shows the LED power consumption over time. In this case, the driving current is increased to compensate for the light power that is reduced over time, as described above. The increase of the driving current causes increased heat generation, the increased heat generation causes the temperature to increase, and as a result, a vicious cycle of increasing the driving current to compensate for the light power that is reduced due to the temperature increase is repeated. Accordingly, the probability that a sudden failure of the LED occurs is heightened.
To mitigate the above-described problems, a limit value of the driving current may be set. However, in an RGB time-sequential type projector, if the driving current of any one color reaches the limit value, the light power of that color become relatively insufficient to cause a problem of white point distortion. For example, when a limit value for the R (Red) color current is reached, the light power for the R color will not be further increased, and thus a blue shift problem that the white point is shifted to the Blue (B) color may occur during the additional increase of the operation temperature.